Tank Air Expansion On Ascent?

[BarryNL]

Dont know about JValve... what I do know is that 'getting hard to inhale' should not be a measure of how much gas you have and definitely not be be a surprise.....

How much harder it gets (or not) depends on the regulator. Modern regulators don't get harder to breathe until you're sucking your last couple of breaths. Further, it depends on the first stage design - downstream firsts get harder to breathe as the tank empties but upstream firsts actually get easier.

Bottom line is: relying on breathing resistance is not a good gas management strategy.

 

[Thalassamania]

This is getting confused; let’s see if we can sort it out all at once.

J-valves were designed to close off the air supply at a preset pressure and then release the air that was closed off through the actuation of a lever. The way this worked was quite simple. There was an upstream valve with a spring. There were three states that the valve could be in:

• Held open by a mechanical lever – mechanism deactivated.
• Held open by air pressure.
• Closed - when the pressure in the tank dropped enough for the spring to close the valve against the air pressure the air supply was interrupted until the lever was pulled (state 1 above).

An SPG would read normally until state 3 was reached, at which time it would preciously drop as the high pressure air dropped when the valve was shut by the spring.

The type of regulator had nothing to do with any of this, upstream, downstream, piston, diaphragm; all are irrelevant to this question.

The exact pressure at which the J-valve cut off the air supply was dependent upon the strength of the spring. Originally 300 lb and 500 lb springs were available. As they fatigued the actual pressure of the air held back lessened. Holding back air also depended on the quality of the valve and its seat. Any damage to either resulted in air not being contained and leaking past the valve even when it was in the closed position. This was a common problem and, combined with availability of the SPG, a contributing cause to the J-valves demise.

 

[Rick Murchison]

Any damage to either resulted in air not being contained and leaking past the valve even when it was in the closed position. This was a common problem and, combined with availability of the SPG, a contributing cause to the J-valves demise.

These problems together with inadvertant actuation of the valve lever are largely why most of the folks I knew - including me - just used K valves. With a Conshelf it was no problem telling when the pressure dropped below 150 psi and it was time to head for the surface. At 60 fpm and no stops that was plenty. The only time anyone got in trouble was when they tried "just one more shot." These are the same folks who get in trouble with properly operating J valves or SPG's
Rick
P.S. Oh, yeah, then there was the most irritating problem of all... the old "filled with the lever up" empty tank and subsequent ruined weekend.

 

[Devil505]

Thalassamania's post covers it all. Thanks!

 

[Thalassamania]

Thanks, but Rick's codicil is an important one that I'd forgotten. Whenever you had a J-valve tank filled you need to check it, as Rick pointed out. What a PITA.